Thermoplastic hot melt adhesives have been used in a wide variety of applications for many years. These formulations have generally been thermoplastic materials which are coated from a heated melt and solidify when cooled to produce a uniform coating of the adhesive, which subsequently can be softened to permit other materials to be bonded thereto when the adhesive cools and solidifies. Adhesion is thereby achieved by an entrapment mechanism whereby the fluid thermoplastic adhesive, having the proper tacky characteristics, wets the material to be adhered and then solidifies on cooling to form a mechanical bond or a chemical and mechanical bond between the materials.
Solidification of the adhesive by virture of its cooling can be achieved by cooling the materials being contacted through the adhesive either directly or by permitting the adhesive to lose heat to the materials to be bonded. In either case, the coating and cooling characteristics of the adhesive are generally controlled by careful temperature control of the materials to be bonded, during the process.
In summary, hot melt adhesives achieve their bonding characteristics through cooling as contrasted with other adhesives which achieve adhesion through a solid adhesive by evaporation of a solvent carrier or through other mechanisms such as polymerization.
The ideal hot melt adhesive should be solid and not tacky, at the temperatures which the coated materials or bonded materials would encounter in normal use and storage. This is important where paper products are coated with a hot melt adhesive prior to their use in a bonding process and are thereafter stored for example in a rolled form. Normally a Kraft paper is coated on one side with the hot fluid adhesive in a conventional manner and cooled to a solid layer prior to rolling the coated paper on itself. This places the adhesive layer in contact with the uncoated surface of an adjacent layer of paper. Any tendency of the thermoplastic adhesive to bond to adjacent sheets on the roll under the conditions encountered during the storage of a prepared roll would enable a bond to form between the layers of paper on the roll creating a condition called "blocking". This term aptly describes the undesirable result of a roll of adhesive coated paper which has bonded to the adjacent layers in the roll forming a solid essentially unuseable product. In addition the coating on the paper must be capable of being reheated, as by placing it in contact with a heated material to be bonded to the paper, to make the adhesive tacky for subsequent adhesion of the materials to be bonded when the adhesive solidifies. During such a procedure there should not be any attendant unwanted migrations of the adhesive either on the coated face of the paper or through the paper by wicking action. This latter requirement is also applicable to the initial coating of the paper with the fluid adhesive.
The kind and amounts of constituents used in hot melt adhesive formulations have varied widely depending upon the physical characteristics of the raw materials, the intended use or application of the adhesive and economic or cost considerations. Generally, suitable formulations for bonding paper products to glass fiber mats, batts or blankets have included thermoplastic polymer resins in combinations with microcrystalline waxes, paraffin waxes, and hydrocarbon resins. Suitable polymers have included polyethylene, polyvinyl acetate, ethylene-vinyl acetate (EVA) and ethylene-ethylacrylate (EEA) co-polymers.
It has been recognized that for this application there are several additional characteristics to these required for other applications, which are necessary for hot melt adhesives to exhibit when attempting to formulate particular adhesives useful for bonding glass fibers to paper. It is not sufficient to evaluate merely the coating ability, bonding strength or the like. Typically, the viscosity of the thermoplastic adhesive in the fluid state at the temperature of application or the temperatures encountered during the process of adhering one material to another, is extremely important for several reasons. The properties known as adhesive tack and the wetting ability of the fluid melt on the material to be coated or adhered, are related to viscosity at the temperatures encountered during application. This can have profound effect on both the initial adhesion to the paper and the ultimate bonding strength. This property will then establish the suitability of the adhesive for use with particular materials. For example, formulations which produce very low viscosity thermoplastic adhesive melts at the temperatures of application or use can produce an undesirable wetting action on a paper substrate such that the fluid adhesive can bleed through the paper. This diminishes its availability for subsequent adhesion when another material is pressed against its adhesive coated face. In addition this bleed through, will produce undesirable asthetic qualities in the final paper faced product. While such a wicking or bleeding through of a waxed based formulation may be desirable in the production of waxed paper, it is undesirable if it occurs in a non-uniform fashion or deprives the adhesive coated surface of the paper of a sufficient adhesive coating for accomplishing the desired mechanical bonding upon cooling or solidification of the adhesive. Generally, the inclusion of higher molecular weight polymers in the thermoplastic adhesive formulation will tend to produce higher viscosity melts at given temperatures of use.
With the foregoing in mind it is important that a balance must be struck when selecting components and proportions of components in any given formulation. Selection of a polymer of a given molecular weight to produce ease of application at a given temperature level may adversely affect the ability of the final formulation to provide a sufficiently sticky or tacky property for particular applications. This last property can be improved by addition of other components, sometimes termed tackifiers, in combination with miscible diluent waxes and fillers or extenders. In this manner, the diluent system provides a variety of effects depending on the kinds of diluents and their concentration in a particular formulation. Generally the diluent acts as a vehicle for the polymer and tends to lower the melt viscosity of the hot melt at the temperatures employed to make it more covenient to apply. In addition, if properly formulated into the hot melt compositions, diluents can enhance the wetting ability and adhesive strength of the polymer in addition to providing the required tack or tackiness or tack range for the adhesive in the fluid or semi-fluid state at the temperatures encountered during coating and use.
Important attributes are also provided to the overall combination of materials from the selection of the other individual components and their proportions. The water vapor barrier property of the adhesive in the finished product is also an important property for the herein described application.
In general, it is believed that the polymer component of a hot melt adhesive formulation forms the back bone of the composition but the exact properties of the final product cannot be predicted with accuracy from the selection of the polymer alone because of the contrubtions made by the other components to all the essential properties required of the hot melt product in use. Therefore, the molecular weight and concentration of the polymer in the system may vary considerably depending on the other materials present and the ability of the system to exhibit the desired properties can often be achieved only after extensive experimentation. It is not possible to merely select components and proportions of components according to some well defined lows or prior empirical experience when preparing a new formulation for achieving specific and different properties.
In building insulating materials, the thermoplastic adhesive, ideally, should act as both an adhesive and a barrier to transmission of the water vapor in the air through the finished product. If the hot melt adhesive formulation selected does not exhibit this property in both the creased and uncreased condition, other materials would then have to be incorporated into the product to provide this necessary property thereby increasing the cost of the resultant product to the consumer.
It is therefore an object of the present invention to provide a low cost hot melt adhesive suitable for adhering a paper facing to fiberous thermal insulating material, such as glass fiber insulating materials, which is capable of being applied, without undersirable bleeding through the paper, at the temperatures normally encountered both during coating and at the point in the manufacture of glass fiber insulating material where a paper facing is applied to the fiber glass insulating material. It is also an object of the present invention to provide such a hot melt adhesive composition which eliminates the current need for supplemental cooling of the glass fiber insulating product prior to the step of adhering the fibers to the adhesive coated face of a paper backing material. It is also an object of the present invention to provide in the final product a water vapor barrier which is superior to that obtained from similar hot melt adhesives capable of only being applied at a lower temperatures i.e. after cooling the glass fiber insulating material.